A power steering device has been widely used as a device for reducing power required for a driver to operate a steering wheel when the driver gives a steering angle to a steering wheel (excluding special vehicles such as forklifts, normally a front wheel). In addition, as this steering device, an electric power steering device which uses an electric motor as an auxiliary power unit has come into use in recent years. Compared to a hydraulic power steering device, the electric power steering device can adopt a miniaturized and lightweight configuration, thereby providing advantages in that not only is a magnitude (torque) of auxiliary power easily controlled but also an engine is subjected to energy loss less.
Various strictures are known in the electric power steering device. In a case of any structure, the auxiliary power of the electric motor is transmitted via a reduction gear to a steering shaft rotated by the operation of the steering wheel, or to a member (a pinion shaft or rack shaft which configures a steering gear unit) which is displaced in response to the rotation of the steering shaft. As this reduction gear, a worm reduction gear is generally used. In a case of the electric power steering device using the worm reduction gear, the auxiliary power of the electric motor is freely transmitted to a rotary shaft which is an output unit of the worm reduction gear in such a way that a worm rotatably driven by the electric motor meshes with a worm wheel rotating with the rotary shaft.
For example, Patent Document 1 discloses an electric power steering device as illustrated in FIGS. 22 and 23. In the electric power steering device, a front end portion of a steering shaft 2 serving as a rotary shaft, which is rotated in a predetermined direction by a steering wheel 1, is rotatably supported inside a housing 3, and a worm wheel 4 is fixed to the front end portion of the steering shaft 2. In addition, in a state where a worm gear 5 disposed in an axially intermediate portion of a worm shaft 6 rotatably driven by an electric motor 7 meshes with the worm wheel 4, a base portion of the worm shaft 6 is rotatably supported by a base side bearing 8, and similarly a tip portion thereof is rotatably supported by a tip side hearing 9 inside the housing 3, respectively.
In the worm reduction gear including the worm wheel 4 and the worm gear 5 which mesh with each other, inevitable backlash is usually present in a meshing portion between the worm wheel 4 and the worm gear 5. The backlash occurs not only due to a dimensional error or an assembling error of each member configuring the worm reduction gear, but also due to abrasion on a tooth surface between the worm wheel 4 and the worm gear 5. In particular, in recent years, the auxiliary power tends to increase. Consequently, the abrasion amount increases, and thus the backlash is likely to occur. In any case, if the backlash is present in the meshing portion, when a rotation direction of the steering shaft 2 is changed or when rotational vibrations are applied from a wheel side to the steering shaft 2, there is a possibility that harsh gear rattling noise may be generated in the meshing portion.
Therefore, in a case of the illustrated structure, in order to minimize the backlash in the meshing portion between the worm wheel 4 and the worm gear 5, the worm shaft 6 is caused to oscillate around the base side bearing 8 so that the worm gear 5 is biased toward the worm wheel 4.
For this reason, in the case of the illustrated structure, a holding recess 10 is disposed in a peripheral portion of a tip portion of the worm shaft 6 inside the housing 3, and a holder 11 is held and fixed inside the holding recess 10. In addition, an outer ring configuring the tip side bearing 9 is internally fitted and fixed to the holder 11, and an annular bush 12 made of an elastic material is internally fitted and fixed to an inner ring configuring the tip side bearing 9. Then, a tip side portion of the worm shaft 6 is loosely inserted into the bush 12. In this manner, the tip side portion of the worm shaft 6 is supported by the holder 11 so as to be rotatable and movable close to or away from the worm wheel 4. In addition, a preload pad 13 is disposed in a portion adjacent to an axially outer side (right side in FIG. 23) of the holder 11 inside the holding recess 10 so as to enable displacement in a meshing direction (vertical direction in FIG. 23) between the worm wheel 4 and the worm gear 5. Concurrently, a tip portion of the worm shaft 6 is inserted into a through-hole disposed in a central portion of the preload pad 13 so as to be rotatable relative to the preload pad 13 without any rattling in a radial direction. Then, resilience of a coil spring 14 installed between the preload pad 13 and the holder 11 presses the tip portion of the worm shaft 6 toward the worm wheel 4 via the preload pad 13. This causes the worm shaft 6 to oscillate around the base side bearing 8. In this manner, the worm gear 5 is biased toward the worm wheel 4, thereby minimizing the backlash in the meshing portion between the worm gear 5 and the worm wheel 4, and suppressing the occurrence of the gear rattling noise in the meshing portion.
A meshing reaction force applied to the worm shaft 6 from the meshing portion between the worm wheel 4 and the worm gear 5 includes not only a component in the meshing direction (vertical direction in FIG. 23) but also a component in directions (forward and rearward directions in FIG. 23) respectively perpendicular to the meshing direction and the axial direction of the worm shaft 6. Hereinafter, this point will be described with reference to FIGS. 24 to 26.
As illustrated in FIGS. 24 and 25, if a driving force is transmitted from the worm shaft 6 to the worm wheel 4 by driving the worm shaft 6 rotatably, the meshing reaction force is applied from the worm wheel 4 to the worm shaft 6. In a case illustrated in FIG. 24 and in a case illustrated in FIG. 25, the driving forces applied to the worm shaft 6 have the same magnitude as each other. However, rotation directions of the driving forces are opposite to each other. Therefore, the worm wheels 4 in the case illustrated in FIG. 24 and in the case illustrated in FIG. 25 rotate in the directions opposite to each other. In this state, in the meshing portion between the worm wheel 4 and the worm gear 5, the apparent meshing reaction force having component forces Fx, Fy, and Fz which are respectively components in three directions x, y, and z in FIGS. 24 and 25 is applied from the worm wheel 4 to the worm shaft 6. Among these component forces Fx, Fy, and Fz, the component forces Fx and Fz are applied in the directions opposite to each other in a case where the worm wheel 4 rotates in one direction {a direction illustrated by an arrow A in FIG. 24(A)} as illustrated in FIG. 24 and in a case where the worm wheel 4 rotates in the other direction {a direction illustrated by an arrow B in FIG. 25(A)} as illustrated in FIG. 25.
On the other hand, when a distance between the meshing portion and an oscillation center o of the worm shaft 6 in the radiation direction of the worm shaft 6 is set to d6, a moment M having a magnitude of d6·Fx is applied to the worm shaft 6. Therefore, when a distance between the meshing portion and the oscillation center o in the axial direction of the worm shaft 6 is set to L6, a force Fr having a magnitude of M/L6 based on the moment M is applied in the radial direction (upward direction in FIG. 24, downward direction in FIG. 25) of the worm shaft 6. The forces Fr are applied in the directions opposite to each other in a case illustrated in FIG. 24 and in a case illustrated in FIG. 25. Therefore, a magnitude of an actual force Fy′ in the direction y which is applied from the worm wheel 4 to the worm shaft 6 in the meshing portion and which considers the moment M decreases (becomes Fy′=Fy−Fr) when the worm wheel 4 rotates in one direction as illustrated in FIG. 24, and increases (becomes Fy′=Fy+Fr) when the worm wheel 4 rotates in the other direction as illustrated in FIG. 25. Accordingly, a resultant force F′ of the actual meshing component forces applied to the mashing portion in the directions y and z decreases as illustrated by an arrow C in FIG. 26 when the worm wheel 4 rotates in one direction, and increases as illustrated by an arrow D in FIG. 26 when the worm wheel 4 rotates in the other direction. Then, as is understood from the direction of the resultant force F′, even when the worm wheel 4 rotates in any direction, the meshing reaction force applied from the meshing portion to the worm shaft 6 includes components in the meshing direction (vertical direction in FIGS. 24 to 26) between the worm wheel 4 and the worm gear 5, and the directions {forward and rearward directions in FIGS. 24(A) and 25(A), rightward and leftward directions in FIGS. 24(B), 25(B), and 26} perpendicular to the axial direction of the worm shaft 6.
In a case of the above-described electric power steering device in the related art, the worm shaft 6 has a portion for externally fitting the preload pad 13 on a further tip side from a portion supported by the tip side bearing 9. Therefore, an axial dimension of the tip side portion of the worm shaft 6 increases correspondingly. Consequently, the worm reduction gear configured to include the worm shall 6 is less likely to be miniaturized.